Modulation method for direct current (DC) coupling converter with less dc voltage sensors

Abstract

A modulation method for a direct current (DC) coupling converter with less DC voltage sensors includes: obtaining a voltage modulated wave v r * and a current reference value i g * of the DC coupling converter; performing determination on the voltage modulated wave v r * and the current reference value i g *, and determining a switch state of the DC coupling converter based on a result of the determination; and when a DC voltage sensor of an electrolytic capacitor of an intermediate power module is removed, modulating a DC voltage sensor based on the voltage modulated wave v r * and a difference between DC voltages on electrolytic capacitors of the other two power modules. When the intermediate DC-side voltage sensor is removed, DC voltages of various modules of a converter system of a DC coupling power unit can still be balanced, and quality of a grid-connected current is not affected by a fault.

The invention claimed is: 
     
       1. A modulation method for a direct current (DC) coupling converter with less DC voltage sensors, wherein the DC coupling converter comprises three power modules, and the modulation method specifically comprises the following steps:
 step 1: obtaining a voltage modulated wave v r * and a current reference value i g * of the DC coupling converter; 
 step 2: performing determination on the voltage modulated wave v r * and the current reference value i g * in step 1, and determining a switch state of the DC coupling converter based on a result of the determination; and 
 step 3: when a DC voltage sensor of an electrolytic capacitor of an intermediate power module is removed, modulating a DC voltage sensor based on the voltage modulated wave v r * and a difference between DC voltages on electrolytic capacitors of the other two power modules; 
 wherein in step 1, the obtaining a voltage modulated wave v r * and a current reference value i g * specifically comprises: 
 step 11: obtaining a phase signal θ of a power grid by performing phase locking on a voltage v g  of the power grid, wherein the three power modules of the DC coupling converter are a power module 1, a power module 2, and a power module 3, and DC-side voltages of the power module 1, the power module 2, and the power module 3 are v dc1 , v dc2 , and v dc3  respectively; 
 step 12: performing aβ/dq transformation on phase signals θ of the voltage v z  of the power grid and a current i g  of the power grid to obtain an active component v d  and a reactive component v q  of the v z , and an active component i d  and a reactive component i q  of the i g ; 
 step 13: converging, by an average DC voltage control loop of a DC link of the DC coupling converter, an average value v dcavg  of an outer loop voltage to a reference value v dcref , after performing a subtraction on the reference value v dcref  and the average value d cavg , obtaining, by using a proportional Integral (PI) controller, a reference value i dref  of the active component of the current i g  of the power grid; and obtaining the current reference value i g * after making the i dref  and a phase of the power grid sinusoidal; 
 step 14: after performing a differential operation on the i dref  and the i d , obtaining an active power error i derr  of the current by using the PI controller, and adding up the i derr , the v d , and i q *ωL to obtain a d-axis component v dref  of the voltage modulated wave, wherein ω represents an angular frequency of the power grid, and L represents grid-connected filtering inductance; 
 step 15: obtaining by using the PI controller, a reactive power error i qerr  of the current based on a difference between i qref  and i q , and adding up the i qerr , the v q , and i d *ωL to obtain a q-axis component v qref  of the voltage modulated wave; and 
 step 16: performing dq/αβ transformation on the v dref  and the v qref  to obtain the voltage modulated wave v r *; 
 wherein in step 2, the performing determination on the voltage modulated wave v r * and the current reference value i g *, and determining a switch state of the DC coupling converter based on a result of the determination specifically comprises: 
 when v r *>0 and i g *>0, determining that the v r * and the i g * are in an operation range I; 
 when v r *<0 and i g *>0, determining that the v r * and the i g * are in an operation range II; 
 when v r *<0 and i g *<0, determining that the v r * and the i g * are in an operation range III; or 
 when v r *>0 and i g *<0, determining that the v r * and the i g * are in an operation range IV, wherein 
 switch states are I1 to I6 in the operation range I, II1 to II6 in the operation range II, III1 to III6 in the operation range III, and IV1 to IV6 in the operation range IV; 
 wherein in step 3, when a voltage sensor of the power module 2 is faulty, the DC voltage sensor is modulated based on the voltage modulated wave v r * and a difference between the v dc1  and the v dc3 , which specifically comprises: 
 in the operation range I, obtaining the difference between the v dc1  and the v dc3  according to Δv dc =v dc1 −v dc3 ; and when 0<v r *<1, selecting switch states I1 and I3 for switching when Δv dc >0, or selecting switch states I2 and I3 for switching when Δv dc <0; when 1<v r *<2, selecting switch states I4 and I3 for switching when Δv dc >0, or selecting switch states I5 and I3 for switching when Δv dc <0; or when 2<v r *<3, selecting switch states I4 and I6 for switching when Δv dc >0, or selecting switch states I5 and I6 for switching when Δv dc <0. 
 
     
     
       2. The modulation method for a DC coupling converter with less DC voltage sensors according to  claim 1 , wherein in step 3, when a voltage sensor of the power module 2 is faulty, the DC voltage sensor is modulated based on the voltage modulated wave v r * and a difference between the v dc1  and the v dc3 , which specifically comprises:
 in the operation range II, when −1<v r *<0, selecting switch states II1 and II3 for switching when Δv dc >0, or selecting switch states II2 and II3 for switching when Δv dc <0; when −2<v r *<−1, selecting switch states II4 and II3 for switching when Δv dc >0, or selecting switch states II5 and II3 for switching when Δv dc <0; or when −3<v r *<−2, selecting switch states II4 and II6 for switching when Δv dc >0, or selecting switch states II5 and II6 for switching when Δv dc <0. 
 
     
     
       3. The modulation method for a DC coupling converter with less DC voltage sensors according to  claim 1 , wherein in step 3, when a voltage sensor of the power module 2 is faulty, the DC voltage sensor is modulated based on the voltage modulated wave v r * and a difference between the v dc1  and the v dc3 , which specifically comprises:
 in the operation range III, when −1<v r *<0, selecting switch states III1 and III3 for switching when Δv dc >0, or selecting switch states III2 and III3 for switching when Δv dc <0; when −2<v r *<−1, selecting switch states III4 and III3 for switching when Δv dc >0, or selecting switch states III5 and III3 for switching when Δv dc <0; or when −3<v r *<−2, selecting switch states III4 and III6 for switching when Δv dc >0, or selecting switch states III5 and III6 for switching when Δv dc <0. 
 
     
     
       4. The modulation method for a DC coupling converter with less DC voltage sensors according to  claim 1 , wherein in step 3, when a voltage sensor of the power module 2 is faulty, the DC voltage sensor is modulated based on the voltage modulated wave v r * and a difference between the v dc1  and the v dc3 , which specifically comprises:
 in the operation range IV, when 0<v r *<1, selecting switch states IV1 and IV3 for switching when Δv dc >0, or selecting switch states IV2 and IV3 for switching when Δv dc <0; when 1<v r *<2, selecting switch states IV4 and IV3 for switching when Δv dc >0, or selecting switch states IV5 and IV3 for switching when Δv dc <0; or when 2<v r *<3, selecting switch states IV4 and IV6 for switching when Δv dc >0, or selecting switch states IV5 and IV6 for switching when Δv dc <0.